The present invention relates to holding loose optical fibers and, more particularly, to an optical fiber holding structure that restrains multiple layers of loose fibers or protects a fiber splice within a cable joint.
Optical fibers are deployed across many miles of ocean and other large bodies of water to establish communication networks. For protection in an undersea environment, optical fibers can be contained within a protective cable. According to one type of prior art fiber optic cable 2, FIG. 1, the optical fibers F are contained within a loose tube 6, which is surrounded by protective layers 8. The protective layers 8 typically include an outer insulating plastic coating, a copper sheath inside the outer coating, and high strength members, such as steel wires, inside of the copper sheath. This type of cable 2 is useful in high strength applications and permits the fibers F to move with respect to the protective layers 8.
Because these fiber optic cables cannot be made and deployed in infinite lengths, sections of the cables must be attached together to extend across large distances. A splicing process is typically used to attach the optical fibers. To protect the splices from the environment, the spliced sections are commonly contained in a watertight housing, often referred to as a joint box. A joint box may also be used when making splices in the field, for example, when a cable must be repaired.
The cable joint is often subjected to substantial tension applied to the cable, for example, during deployment of cable into the sea by a ship. The joint box is intended to prevent the applied tension forces from being absorbed by the fiber splice. Because the optical fibers are able to move within the cable, the joint box is also intended to prevent the spliced portion from being pulled into the cable. In one prior joint box design, the high strength steel wires are clamped to a first end of a joint box between a socket body and a plug and sleeve arrangement, as disclosed in U.S. Pat. No. 4,507,008, entitled STRANDED CABLE TERMINATION ARRANGEMENT and incorporated herein by reference. The fibers extend unrestrained through the hollow plug and sleeve to a center section or shelf of the joint box, and the ends of the fibers are spliced. The joined fibers are coiled onto the center section with a large amount of slack (e.g., about three feet) and with the fiber splice fixed within the center section. The large amount of slack enables the spliced optical fibers to be pulled or tensioned without placing stress on the fiber splice and without the fiber splice being pulled into one of the cables.
However, this joint box design has some drawbacks. For example the large amount of excess slack may be undesirable from an assembly standpoint. Further, when the excess slack is bent to fit within the housing, the fibers could become damaged. If the excess slack is removed, the stresses and strains will be applied at the splice affixed within the housing.
Accordingly, there is a need for an optical fiber holding structure that restrains loose fibers in a cable joint and prevents having to use a large amount of excess slack fiber within the joint box. Although existing splice protection devices may be capable of acting as a fiber holding structure, such devices are not capable of accommodating high fiber counts without causing damage (e.g., microbending) to the fibers. Thus, there is also a need for an optical fiber holding structure capable of restraining high fiber count fibers or providing splice protection for high fiber counts.
One aspect of the present invention is an optical fiber holding structure for holding at least first and second layers of optical fibers. The optical fiber holding structure comprises an outer gripping tube, a semi-rigid splint member positioned within the outer gripping tube, and a compliant splint member positioned within the outer gripping tube. The first and second layers of optical fibers preferably pass between the semi-rigid splint member and the compliant splint member. The outer gripping tube causes the first and second layers of fibers to be gripped between the semi-rigid splint member and the compliant splint member. The optical fiber holding structure can be used as a fiber restraint where the optical fibers pass continuously through the holding structure or as a splice protection device where the optical fibers are spliced within the holding structure.
According to one preferred embodiment, the outer gripping tube is a heat shrink tube, and each layer of fibers is ribbonized and surrounded by adhesive. The semi-rigid splint member and the compliant splint member preferably have a generally semi-cylindrical shape. The semi-rigid splint member is preferably made of glass, and the compliant splint member is preferably made of plastic.
According to one embodiment, an internal support is positioned between the first and second layers of optical fibers. One embodiment of the internal support is made of a compliant material. Another embodiment of the internal support includes a rigid beam, such as an I-channel, an H-channel, or a rectangular channel, coated with a compliant material.
According to another embodiment, a third layer of optical fibers extends through the outer gripping tube between the splint members. A compliant internal support is positioned between the first and third layers of optical fibers, and a semi-rigid internal support is positioned between the second and third layers of optical fibers. According to a further embodiment, the first and second layers of fibers are held without any internal support.
According to a further aspect of the present invention, the optical fiber holding structure is provided as an assembly comprising the outer gripping tube, the splint members, and at least one internal support adapted to be assembled for holding layers of optical fibers.
Another aspect of the present invention is a method of holding optical fibers. The method comprises: inserting a compliant splint member and a semi-rigid splint member within a heat shrink tube; inserting first and second layers of optical fibers into the heat shrink tube and between the compliant splint member and the semi-rigid splint member; and heating the heat shrink tube containing the compliant splint member and the semi-rigid splint member and the optical fibers to form an optical fiber holding structure. The method also preferably includes the step of inserting the first and second layers of fibers in hot melt glue tubes.
One method further includes ribbonizing the optical fibers in each of the layers of optical fibers. Another method further includes splicing fibers together in each of the layers of optical fibers.